This invention relates to a process for preparing dialkyl carbonates and diols. More specifically the present invention relates to an integrated process for preparing dialkyl carbonates and diols from alkylene oxides, carbon dioxide and alcohols.
Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, which use diphenyl carbonate and bisphenol-A as starting materials.
Dimethyl carbonate has a low toxicity and can also be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaterniation of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e. the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates.
Non-phosgene methods that have been proposed for producing dialkyl carbonates include the transesterification reaction of alcohols and cyclic carbonates. Most of the proposed methods relate to the use of various catalysts for that reaction. Examples of such proposed catalysts include alkali metals or basic compounds containing alkali metals; tertiary aliphatic amines; thallium compounds; tin alkoxides; alkoxides of zinc, aluminum and titanium; a mixture of a Lewis acid and a nitrogen-containing organic base; phosphine compounds; quaternary phosphonium salts; cyclic amidines; compounds of zirconium, titanium and tin; a quaternary ammonium group-containing strongly basic anion-exchange solid material; a solid catalyst selected from the group consisting of a tertiary amine-or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a mixture of an alkali metal with silica, a silicate of an alkaline earth metal and an ammonium ion-exchanged zeolite; and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound.
With respect to the method for carrying out the above-mentioned transesterification reaction between a cyclic carbonate and an alcohol, the proposed methods generally rely upon using commercial grade reactants (i.e., the cyclic carbonate and the alcohol) having a relatively high purity to achieve adequate reaction rate, yield and selectivity to the desired dialkyl carbonates and diols, as discussed more filly below.
It is known to react alkylene oxides with CO2 in the presence of catalysts to give cyclic carbonates. The proposed methods for this reaction again mostly relate to the development of various catalysts. Examples of such proposed catalysts include alkali metal halides; ammonium, phosphonium or sulphonium halide carbonates; a combination of protic compounds, such as alcohols, and nitrogen-containing bases; arsonium halides; tertiary phosphines with alcohols; and alkali metal transfer catalysts with crown ethers and other ligands.
Although some of these proposed methods for producing cyclic carbonates will provide relatively high yields and selectivity to the desired cyclic carbonate, inevitably a significant amount of glycols are produced as by-products of the reaction between the alkylene oxides and CO2. For example, the reaction between ethylene oxide and CO2 to produce ethylene carbonate will inevitably produce a certain amount of ethylene glycol and higher molecular weight glycols, e.g. di- and tri-ethylene glycol. It is generally believed that the presence of these glycol impurities contained in the cyclic carbonate will adversely affect the transesterification reaction between the cyclic carbonate and an alcohol, since certain glycols, e.g. ethylene glycol, will typically unfavorably affect the equilibrium of the reaction, thereby lowering yield or possibly selectivity for the desired products. In addition, the glycols have hydroxyl (xe2x80x94OH) groups which would be expected to compete with the alcohol in reacting with the cyclic carbonate, forming unwanted heavier carbonates and other undesired species. Moreover, since the glycol impurities, e.g. ethylene glycol and triethylene glycol, are generally hygroscopic in nature, the cyclic carbonate containing these glycol impurities will tend to absorb water during storage. When water is present in sufficient quantity in the reaction mixture, hydrolysis takes place simultaneously with the transesterification reaction, resulting in a decrease in the selectivity for the dialkyl carbonate. Thus, in order to avoid the problems associated with the glycol impurities, the proposed methods for producing dialkyl carbonates from a cyclic carbonate have required a relatively high purity cyclic carbonate.
In order to provide a high purity commercial grade cyclic carbonate, e.g. ethylene carbonate, useful as a reactant in connection with the proposed methods discussed above, difficult separations have to be performed to achieve the requisite cyclic carbonate purity, resulting in increased operating and capital costs. Typically, the purification process for a cyclic carbonate, e.g. ethylene carbonate, produced by reacting an alkylene oxide with CO2, will include the following: (1) flashing the carbonation reactor effluent to remove the light ends; (2) passing the remaining liquid through one or more evaporators to remove and recycle catalyst, and to remove heavies; (3) distilling the crude product stream containing the cyclic carbonate and mono- and poly-glycols in a first vacuum distillation column, containing between about 8 and 50 trays, to remove the majority of the mono- and poly-glycols; and (4) distilling the purified cyclic carbonate stream in a finishing vacuum distillation column, containing between about 8 and 50 trays, to remove remaining polyglycols and other heavies and to provide a high purity cyclic carbonate.
There are significant capital and operating costs associated with the two vacuum distillation columns needed to provide a high purity cyclic carbonate. Moreover, a significant amount of the cyclic carbonate is generally lost as a result of the purification process. For example, in the case of ethylene carbonate, an azeotrope is formed in the first vacuum distillation column, which can contain about 91% ethylene glycol and 9% ethylene carbonate, depending upon the pressure in the column, so that a significant amount of ethylene carbonate is removed with the ethylene glycol. Moreover, depending upon the operating conditions of the distillation columns, and specifically the high temperatures associated with full reboiled columns, a certain amount of decomposition of the cyclic carbonate can occur resulting in additional losses.
As can be understood from the above, no proposal has heretofore been made with respect to an integrated process for producing a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an alcohol, having high productivity, which first reacts an alkylene oxide with carbon dioxide in the presence of a carbonation catalyst to provide a cyclic carbonate, which contains by-product impurities, and then uses the cyclic carbonate containing the impurities as a feedstock for a second transesterification reaction.
According to the present invention, it has now been found that a dialkyl carbonate and a diol, and more specifically dimethyl carbonate and ethylene glycol, can be prepared according to an integrated process having high productivity, by:
reacting an alkylene oxide (ethylene oxide in the case of dimethyl carbonate and ethylene glycol) with carbon dioxide in the presence of a carbonation catalyst in a first reaction zone to provide a crude cyclic carbonate stream comprising a cyclic carbonate (ethylene carbonate in the case of dimethyl carbonate and ethylene glycol) and impurities (e.g. ethylene glycol and higher molecular weight glycols in the case of dimethyl carbonate and ethylene glycol);
directing the crude cyclic carbonate stream to a second reaction zone;
reacting the cyclic carbonate (e.g. ethylene carbonate) with an aliphatic monohydric alcohol (methanol in the case of dimethyl carbonate and ethylene glycol), in the second reaction zone in the presence of a transesterification catalyst to provide a crude product stream containing a corresponding dialkyl carbonate (e.g. dimethyl carbonate) and diol (e.g. ethylene glycol); and
recovering the dialkyl carbonate and the diol from the crude product stream.
Preferably, the alkylene oxide is of the formula: 
wherein R1 and R2 independently of one another denote a divalent group represented by the formula xe2x80x94(CH2)mxe2x80x94, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of C1-C10 alkyl group and a C6-C10 aryl group, wherein R1 and R2 can share the same substituent; and
said aliphatic monohydric alcohol is of the formula:
R3xe2x80x94OHxe2x80x83xe2x80x83(II)
wherein R3 is a monovalent aliphatic C1-C12 hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group and a C6-C10 aryl group.
The present invention provides the advantage of continuously producing dialkyl carbonates and diols, such as dimethyl carbonate and ethylene glycol, with relatively high yield and selectivity to the desired dialkyl carbonate and diol, and obtaining the productivity and economic benefits associated with integrating the carbonation and transesterification processes.
Additional objects, advantages and novel features of the invention will be set forth in part in the description and examples which follow, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is a continuous integrated process for preparing dialkyl carbonates and diols from alkylene oxides, carbon dioxide and aliphatic monohydric alcohols.
In preparing the dialkyl carbonates and diols, an alkylene oxide is first reacted with CO2 in the presence of a carbonation catalyst to provide a corresponding cyclic carbonate. Preferably, the alkylene oxide is represented by structural formula (I) above. Examples of such alkylene oxides include ethylene oxide, propylene oxide, styrene oxide, trimethylene oxide, cyclohexene oxide, and the like. Of these alkylene oxides, ethylene oxide and propylene oxide are preferably used because of their good availability and high demand end proproducts. Ethylene oxide is most preferably used.
The first carbonation reaction involving this preferred alkylene oxide may be represented by the following: 
wherein R1 and R2 independently of one another denote a divalent group represented by the formula xe2x80x94(CH2)mxe2x80x94, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of C1-C10 alkyl group and a C6-C10 aryl group, wherein R1 and R2 can share the same substituent.
The carbon dioxide to be employed can contain inert gases, such as nitrogen, hydrogen, carbon monoxide and lower hydrocarbons, and can originate from natural sources or industrial waste gases.
The content and amount of carbon dioxide will depend on the reaction rate, reactor type and specific catalyst used, and is adjusted to maximize the economics of the process. Preferably, the molar ratio of alkylene oxide to carbon dioxide is about 1:1, but an excess of carbon dioxide is also contemplated.
Therefore, according to the invention, the molar ratio of alkylene oxide to carbon dioxide is preferably in the range from about 1:0.9 to 1:15 and more preferably in the range from about 1:1 to 1:3.
The reactants (i.e. alkylene oxide and carbon dioxide) are contacted in the presence of a carbonation catalyst. The carbonation catalyst can typically be any homogeneous catalyst known in the art which provides adequate reaction kinetics. Preferred homogeneous carbonation catalysts include phosphonium halides, arsonium halides, quaternary ammonium halides and alkali halides. The carbonation catalyst can also include heterogeneous catalysts such as, for example, anion exchange resins having quaternary ammonium functional groups or solid support catalysts containing an element of groups IA or IIA of the periodic table, such as those described in application Ser. No. 09/332,725 filed Jun. 14, 1999, which is incorporated herein by reference.
The specific catalyst is chosen to optimize the economics of the overall integrated process and will depend upon the particular alkylene oxide being reacted and the reaction conditions. For example, it is contemplated that tetraethyl ammonium bromide and potassium iodide are particularly effective for the synthesis of ethylene carbonate. The weight ratio of catalyst to alkylene oxide is generally about 0.001:1 to 0.05:1, preferably about 0.003:1 to 0.02:1.
The carbonation reaction is preferably carried out in a continuous mode utilizing various reaction configurations, such as a stirred-tank, tubular, fixed or packed-bed reactor, in a single or multiple-reactor configuration, at from about 50xc2x0 C. up to about 250xc2x0 C., preferably between about 100xc2x0 C. up to about 200xc2x0 C., and at pressures ranging from about atmospheric pressure up to about 14000 Kpa (2000 psi), preferably from about 2000 Kpa ( 300 psi) up to about 9000 Kpa (1300 psi). In the preferred mode of operation, the reactor temperature and pressure are optimized to insure a relatively high conversion and selectivity to the desired cyclic carbonate. A provision for heat removal from the reactor is normally required, since the carbonation reaction is exothermic.
Preferably, the effluent from the carbonation reaction zone is fed into a second carbonation reaction zone that can operate under different conditions or a different configuration to provide a greater overall conversion of the alkylene oxide, preferably greater than 90% overall conversion. Preferably, the second carbonation reaction zone is a separate tubular polishing reactor which operates at from about 50xc2x0 C. up to about 250xc2x0 C., preferably between about 100xc2x0 C. up to about 200xc2x0 C. and at pressures ranging from about atmospheric up to about 14000 Kpa (2000 psi), preferably from about 2000 Kpa (300 psi) to about 9000 Kpa (1300 psi).
Inevitably, impurities are formed in the carbonation reaction in the form of by-products. Typically, glycols are formed along with the cyclic carbonates, especially if there is water present in the system. For example, by reacting ethylene oxide with CO2 to produce ethylene carbonate, inevitably ethylene glycol and some higher molecular weight glycols are produced.
The carbonation reactor effluent, either from the first carbonation reaction zone or from the second carbonation reaction zone (if used), is preferably subjected to a flash separation to remove the volatiles, such as unreacted CO2 and alkylene oxide. In the case of a homogeneous carbonation catalyst, the carbonation reactor effluent will be fed to a separating apparatus to recover the catalyst for recycle to the carbonation reaction zone(s). Examples of such separating apparatuses include an evaporation type separating apparatus, a crystallization type separating apparatus, an absorption type separating apparatus and a membrane type separating apparatus. A combination of a plurality of different or identical separating apparatuses may be used. An evaporation type separating apparatus is especially preferred.
In one embodiment, an evaporator used to recover the homogeneous carbonation catalyst is surmounted by a packed or trayed column rectifying section, having a condenser and reflux. The vaporous effluent from the evaporator, which contains cyclic carbonate and by-product impurities, rises up through the rectifying section where it is condensed and refluxed back down into the evaporator. The condensate effectively flows countercurrent to the rising vapor and helps to wash additional homogeneous catalyst and unwanted heavies from the rising vapor. The crude cyclic carbonate (containing by-product impurities) is recovered overhead of the evaporator as a liquid. In such a configuration, the operating conditions are controlled to minimize decomposition of the cyclic carbonate and the homogeneous catalyst, but to achieve a relatively high recovery of the homogeneous catalyst. As such, the residence time of the carbonation reactor effluent fed to the evaporator/rectifying section is minimized with the reflux ratio relative to the recovered crude cyclic carbonate typically about 0.1 to 0.7. As a comparison, a typical reboiled distillation column has a reflux ratio in excess of 1. The recovered crude cyclic carbonate under such conditions can have between about 0.5 and 40 wt % by-product impurities.
The use of such an evaporator (having a rectifying section) provides improved removal of the homogeneous catalyst and heavies from the crude cyclic carbonate than a typical evaporator, but avoids the high heat and long liquid residence time of a full reboiled distillation column, which can decompose the cyclic carbonate, and the additional costs and yield penalties associated with such a column.
The effluent from the carbonation reaction is preferably not subjected to any further separation. Thus, the cyclic carbonate and the by-product impurities produced in the carbonation reaction are then fed to the second transesterification reaction zone, where the cyclic carbonate is reacted with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a corresponding dialkyl carbonate and diol.
Preferably, the aliphatic monohydric alcohol is represented by structural formula (II) above and has a boiling point lower than that of the produced diol. The type of an aliphatic monohydric alcohol which can be used in the present invention varies depending on the particular cyclic carbonate produced by the carbonation reaction. Examples of such aliphatic monohydric alcohols include methanol, ethanol, n-propanol, iso-propanol, allyl alcohol, butanol (including isomers of butanol), 3-butene-1-ol, amyl alcohol (isomers), hexyl alcohol (isomers), heptyl alcohol (isomers), octyl alcohol (isomers), nonyl alcohol (isomers), decyl alcohol (isomers), undecyl alcohol (isomers), dodecyl alcohol (isomers), cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, methylcyclopentanol (isomers), ethylcyclopentanol (isomers), methylcyclohexanol (isomers), ethylcyclohexanol (isomers), dimethylcyclohexanol (isomers), diethylcyclohexanol (isomers), phenylcyclohexanol (isomers), benzyl alcohol, phenethyl alcohol (isomers), phenylpropanol (isomers), and the like. The above mentioned aliphatic monohydric alcohol may be substituted with at least one substituent, such as a halogen atom, a lower alkoxy group, a cyano group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, a nitro group or the like.
Of these aliphatic monohydric alcohols, an alcohol having 1 to 6 carbon atoms is preferably used. When ethylene carbonate is the cyclic carbonate, an alcohol having 1 to 4 carbon atoms, i.e., methanol, ethanol, propanol (isomers) or butanol (isomers) is preferably used. The method of the present invention can be employed advantageously especially when methanol and ethylene carbonate are used as feedstocks for the transesterification reaction.
According to the present invention, it has now been found that it is unnecessary to purify the cyclic carbonate to achieve relatively high yields and selectivity to the desired dialkyl carbonate and diol, resulting in significant economic benefits and advantage due to the elimination of the purification steps, e.g. two vacuum distillation columns, previously thought necessary. In addition to lower operating and capital costs associated with e liminating the purification steps, a yield benefit is realized by eliminating losses of cyclic carbonate attributable to the purification steps. As such, an integrated process is provided which produces both a dialkyl carbonate and a diol with high productivity. In accordance with the present invention, the term xe2x80x9cproductivityxe2x80x9d means the yield per unit volume of both the carbonation and transesterification zones per unit time, i.e. the space time yield for the overall integrated process.
The transesterification reaction may be represented by the following: 
wherein R1 and R2 in dependently of one another denote a divalent group represented by the formula xe2x80x94(CH2)mxe2x80x94, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of C1-C10 alkyl group and a C6-C10 aryl group, wherein R1 and R2 can share the same substituent; and R3 is a monovalent aliphatic C1-C12 hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group and a C6-C10 aryl group.
The reactants to the transesterification reaction (i.e. the cyclic carbonate and the aliphatic monohydric alcohol) are contacted in the presence of a transesterification catalyst. The transesterification catalyst can typically include any homogeneous or heterogeneous catalyst known in the art which provides adequate reaction kinetics and minimizes side reactions with the impurities contained in the cyclic carbonate.
Examples of such catalysts include alkali metals or alkaline earth metals, such as lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium and the like; basic compounds such as hydrides, hydroxides, alkoxides, aryloxides and amides of alkali metals or alkaline earth metals and the like; basic compounds, such as carbonates and hydrogencarbonates of alkali metals or alkaline earth metal, alkali metal or alkaline earth metal salts of organic acids and the like; tertiary amines such as triethylamine, tributylamine, trihexylamine, benzyldiethylamine and the like; nitrogen-containing heteroaromatic compounds, such as N-alkylpyrrole, N-alkylindole, oxazole, N-alkylimidazole, N-alkylpyrazole, oxadiazole, pyridine, alkylpyridine, quinoline, alkylquinoline, isoquinoline, alkylisoquinoline, acridine, alkylacridine, phenanthroline, alkylphenanthroline, pyrimidine, alkylpyrimidine, pyradine, alkylpyradine, triazine, alkyltriazine and the like; cyclic amidines, such as diazabicycloundecene (DBU), diazabicyclononene (DBN) and the like; thallium compounds, such as thallium oxide, thalliun halides, thallium hydroxide, thallium carbonate, thallium nitrate, thallium sulfate, thallium salts of organic acids and the like; tin compounds, such as tributylmethoxytin, tributylethoxytin, dibutyldimethoxytin, diethyldiethoxytin, dibutyldiethoxytin, dibutylphenoxytin, diphenylmethoxytin, dibutyltin acetate, tributyltin chloride, tin 2-ethylhexanoate and the like; zinc compounds, such as dimethoxyzinc, diethoxyzinc, ethylenedioxyzinc, dibutoxyzinc and the like; aluminum compounds such as aluminum trimethoxide, aluminum triisopropoxide, aluminum tributoxide and the like; titanium compounds, such as tetramethoxytitanium, tetraethoxytitanium, tetrabutoxytitanium, dichlorodimethoxytitanium, tetraisopropoxytitanium, titanium acetate, titanium acetylacetonate and the like; phosphorus compounds, such as trimethylphosphine, triethylphosphine, tributylphosphine, triphenylphosphine, tributylmethylphosphonium halides, trioctylbutylphosphonium halides, triphenylmethylphosphonium halides and the like; zirconium compounds, such as zirconium halides, zirconocenes, zirconium acetylacetonate, zirconium alkoxides, zirconium acetate and the like; lead and lead-containing compounds, e.g., lead oxides, such as PbO, PbO2, Pb3O4 and the like; lead sulfides, such as PbS, Pb2S3, PbS2 and the like; lead hydroxides, such as Pb(OH)2, Pb3O2(OH)2, Pb2[PbO2(OH)2], Pb2O(OH)2 and the like; plumbites, such as Na2, PbO2, K2PbO2, NaHPbO2, KHPbO2 and the like; plumbates, such as Na2PbO3, Na2H2PbO4, K2PbO3,K2[Pb(OH)6], K4PbO4, Ca2PbO4, CaPbO3 and the like; lead carbonates and basic salts thereof, such as PbCO3, PbCO3.Pb(OH)2 and the like; alkoxylead compounds and aryloxylead compounds, such as Pb(OCH3)2, (CH3O)Pb(OPh), Pb(OPh)2 and the like; lead salts of organic acids, and carbonates and basic salts thereof, such as Pb(OCOCH3)2, Pb(OCOCH3)4, Pb(OCOCH3)2.PbO.3H2O, and the like; organolead compounds, such as Bu4Pb, Ph4Pb, Bu3PbCl, Ph3PbBr, Ph3Pb (or Ph6Pb2), Bu3PbOH, Ph2PbO and the like wherein Bu represents a butyl group and Ph represents a phenyl group; lead alloys, such as Pbxe2x80x94Na, Pbxe2x80x94Ca, Pbxe2x80x94Ba, Pbxe2x80x94Sn, Pbxe2x80x94Sb and the like; lead minerals, such as galena, zinc blende and the like; hydrates of these lead compounds; ion-exchangers, such as anion-exchange resins having teriary amino groups, amide groups, or at least one type of ion-exchange group selected from the group consisting of sulfonate, carbonxylate and phosphate groups; strongly basic solid anion-exchangers having quarternary ammonium groups as ion-exchange groups and the like; solid inorganic compounds, such as silica, silica-alumina, silica-magnesia, aluminosilicate, gallium silicate, various types of zeolites, various types of metal-exchanged zeolites, ammonium-exchanged zeolites; and the like.
Preferred homogeneous transesterification catalysts include alcoholates and alkali hydroxides and carbonates, such as sodium methylate and sodium hydroxide. Preferred heterogeneous transesterification catalysts include anion exchange resins having tertiary amine, quaternary ammonium, sulfonic acid or carboxylic acid functional groups or solid support catalysts containing alkaline earth metal halides such as those described in U.S. Pat. No. 5,498,743, which is incorporated herein by reference.
The specific catalyst is chosen to optimize the economics of the overall integrated process and will depend upon the particular cyclic carbonate and aliphatic monohydric alcohol reacted, the type and amount of impurities contained in the cyclic carbonate and the transesterification reaction conditions. For example, it is contemplated that sodium hydroxide and quaternary ammonium anion exchange resins, containing some carbonate or bicarbonate anions, are particularly effective as a transesterification catalyst for the synthesis of dimethyl carbonate and ethylene glycol in accordance with the present invention. The weight ratio of homogeneous catalyst to cyclic carbonate is typically about 0.0005:1 to 0.05:1, preferably about 0.002:1 to 0.01:1. In the case of a heterogeneous catalyst, the weight hourly space velocity (WHSV) will typically be from about 0.1 to about 10, preferably about 0.5 to about 2.
The transesterification reaction is preferably carried out in a continuous mode utilizing various reactor configurations, such as stirred-tank, tubular, fixed or packed-bed reactors, in a single or multiple-reactor configuration, a boiling pot surmounted by a trayed or packed column, or a reactive distillation column, at from about 50xc2x0 C. up to about 250xc2x0 C., preferably between about 75xc2x0 C. up to about 140xc2x0 C., and at pressures ranging from about atmospheric pressure up to about 14000 Kpa (2000 psi), preferably from about 140 Kpa (20 psi) up to about 2000 Kpa (300 psi). In the preferred mode of operation, the type of reactor, temperature and pressure are optimized to insure a relatively high conversion and selectivity to the desired dialkyl carbonate and diol and to optimize the economics of the overall integrated process. Generally, a reactive distillation column will tend to give higher conversions of ethylene carbonate and methanol, while a packed-bed reactor offers flexibility in handling various heterogeneous catalysts.
The effluent from the transesterification reaction will typically be fed to a series of separating apparatuses to recover the dialkyl carbonate and diol products. Examples of such separating apparatuses include a distillation type separating apparatus, an extractive distillation type separating apparatus, a liquid-liquid extraction type separating apparatus, a crystallization type separating apparatus, an absorption type separating apparatus and a membrane type separating apparatus. A combination of a plurality of different or identical separating apparatuses may be used. Among these separating apparatuses, a distillation type separating apparatus is especially preferred. The separated streams resulting from the use of the various separating apparatuses may also be subjected to further processing, such as additional reactions or incorporation into other chemical synthesis processes, as discussed more fully below.